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Chapter 12.docx

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Matthias Niemeier

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Chapter 12
The somatic sensory system
Somatic sensation: Detection of:
1. Touch
2. Pain
3. Temperature
4. Body position
Unique features of the somatic system:
1. Receptors distributed throughout the body
2. Group of 4+ senses rather than one collective sense
3. A single sensory receptor can encode stimulus features such as
a. Intensity
b. Duration
c. Position
d. Direction
4. A single stimulus activates many receptors
5. The central nervous system interprets the activity of the vast receptor array and uses it to
generate coherent perceptions
Two major types of skin:
1. Hairy
2. Glabrous
a. Hairless
Layers of skin:
1. Epidermis
a. Outer layer
2. Dermis
a. Inner layer
Function of skin:
1. Protective function
2. Prevents evaporation of body fluids into the dry environment
3. Largest sensory organ

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Mechanoreceptors of the skin: Makes up the majority of the sensory receptors in the somatic sensory
1. Sensitive to physical distortion such as bending or stretching
2. Monitor contact with the skin
3. Monitor pressure in the heart and blood vessels
4. Monitor stretching of the digestive organs and urinary bladder
5. Monitor Force against the teeth
6. Contains unmyelinated axon branches
a. These branches have mechanosensitive ion channels; their gating depends on changes
of tension (stretching) of the surrounding membrane
7. Vary in their preferred
a. Stimulus frequencies
b. Pressures
c. Receptive field sizes
8. Vary in the persistence of their responses to long lasting stimuli
a. Rapidly adapting
i. Meissner's and pacinian corpuscles tend to respond quickly at first then stop
firing despite continuing stimulus
b. Slowly adapting
i. Merkel's disks and ruffini's endings tend to generate more sustained response
during a long stimulus
Pacinian corpuscle: Largest receptor in the somatic system that lies deep in the dermis (inner layer of
Ruffini's endings: Found in both hairy and glabrous skin. Slightly smaller than Pacinian corpuscles.
Meissner's corpuscles: 1/10 the size of pacinian corpuscles and are located in the ridges of glabrous skin
Merkel's disks: Located within the epidermis, they consist of a nerve terminal and a flattened, non-
neural epithelial cell. The epithelial cell seems to be the mechanically sensitive part because it makes a
synapse like junction with the nerve terminal
Krause end bulbs: Lie in the border regions of dry skin and mucous membrane (around lips and
genitals). The nerve terminals look like knotted balls of string
Skin sensitivity: Skin can be:
1. Vibrated
2. Pressed
3. Pricked
4. Stroked
Skin hairs can be:

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1. Bent
2. Pulled
Receptive field size:
1. Large
a. Pacinian corpuscles
b. Ruffini's endings
2. Small
a. Meissner's corpuscles
b. Merkel's disks
Follicles: The point where hairs grow from. Follicles are embedded in the skin. Each follicle is richly
innervated by free nerve endings that either wrap around it or run parallel to it. There are several types
of hair follicles.
Bending of the hair:
1. Causes a deformation of the follicle and surrounding skin tissues
2. Results in stretching, bending or flattening of the nearby nerve endings
3. Results in the increase or decrease of their action potential firing frequency
4. Mechanoreceptors may be rapidly adapting or slowly adapting
Mechanical sensitivities of mechanoreceptors: Different sensitivities mediate different sensations.
1. Pacinian corpuscles are most sensitive to vibrations of about 200-300 Hz
2. Meissner's corpuscles respond best around 50 Hz
Selectivity of a mechanoreceptive axon depends on the structure of its special ending:
1. The pacinian corpuscle has a football shaped capsule with 20-70 concentric layers of connective
tissue arranged like layers of an onion with the nerve terminal in the middle.
a. When the corpuscle is compressed, energy is transferred to the nerve terminal,
deforming the membrane and opening the mechanosensitive channels.
i. Current flowing through the channels generates a receptor potential which is
1. If the depolarization is large enough, the axon will fire an action
2. The capsule layers contain viscous fluid between them. If the stimulus pressure is maintained,
the layers slip past one another and transfer the stimulus energy in such a way that the axon
terminal is no longer deformed
a. This results in dissipation of the receptor potential
i. When pressure is released, the events reverse themselves and the mechanism
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